This invention relates generally to electronic circuits for detecting the occurrence and magnitude of peaks of amplitude modulated signals, particularly such circuits employed to detect peaks in analog signals read from magnetic disks or tape or optical disks.
State-of-the-art magnetic and optical mass storage systems record data on numerous tracks that are positioned extremely close to one another. It is therefore difficult to maintain a read/write head precisely aligned with a given recording track because of mechanical inaccuracies and instabilities which, although very small, become significant with such close spacing of the tracks. Therefore, servo signals are periodically recorded on each track along with a desired data signal. The known servo signals are read by the head and commands then issued to realign the head with respect to the data track if the detected servo signal is not what is expected.
Such a servo system usually utilizes one or more phase locked loops to synchronize to the signal being read from the track. Certain disadvantages exist with this approach, however. One disadvantage is that phase locked loops require some time to lock onto the incoming signal and this delay can be, in many circumstances, unacceptable. Also, precision phase locked loops need very well controlled filters which typically require external components or very large silicon substrate area integrated circuits. Further, since the drive speed of the media, particularly in the case of tape drives, can vary substantially, a phase locked loop also has to be able to synchronize with signals having a frequency extending over a wide range, thus resulting in the need to repetitively program both the phase locked loop and its filter.
Existing phase locked loop systems use a clock generated by the loop to sample the incoming analog signal at certain time intervals which are assumed to coincide with the amplitude peaks. A measurement of the occurrence and magnitude of peaks constitutes a demodulation of the incoming amplitude modulated analog signal. As the speed of the recording media varies somewhat, however, phase shifts occur in the signal without an immediate correction in the sampling rate, thereby rendering measurements of peaks to be inaccurate. This results in random positioning errors of the read/write head which in turn significantly can increase the error rate of data being written into or being read from the storage system.
Therefore, it is a primary object of the present invention to provide an improved peak detector circuit for a wide variety of applications including the above-described mass storage systems.